1. Field of the Invention
This invention relates generally to instrumentation electronics and more particularly to electronic circuitry which provides a measure of current flow in a circuit.
2. Description of the Prior Art
Techniques for measuring electrical current are well known and cover a wide spectrum of devices and circuits including classical current meters and circuits which add known current-measuring resistance in series with a load, whereupon the voltage drop across the resistor is measured, thus yielding a measure of that current. However, the act of measuring the current often affects the results in that there is a voltage drop across the resistor as well as across the load so that the voltage across the load in effect "droops". If the load is reactive or partially reactive, the measurement also introduces a phase shift in the current. Thus to keep these effects to a minimum, the resistance must be kept relatively small which ultimately reduces the accuracy of the measurement. While high gain instrumentation amplifiers can somewhat compensate for this effect, they also have a tendency to introduce noise which is undesirable and cannot be tolerated in certain applications, such as capacitance type proximity sensors. Accordingly, there is a need to provide current-measuring circuitry which will permit such proximity sensors to be operated relatively close to one another without being affected by cross-talk between them.